Block copolymers as viscosity index improvers for lubricating oils

ABSTRACT

Certain hydrogenated or saturated block copolymers having at least three blocks are highly effective viscosity index improving additives for mineral oils and are especially effective at elevated temperatures.

United States Patent Anderson 1 June 6,1972

[54] BLOCK COPOLYMERS AS VISCOSITY INDEX IMPROVERS FOR LUBRICATING OILS [72] Inventor: William S. Anderson, Oakland, Calif.

[73] Assignee: Shell Oil Company, Houston, Tex.

[22] Filed: Dec. 12, 1969 [21] Appl. No.: 884,720

FOREIGN PATENTS OR APPLICATIONS 1,033,456 6/ 1966 Great Britain Young et a1 ..252/59 X ...252/59 X Shepherd ..252/59 Primary Examiner-Daniel E. Wyman Assistant Examiner-W, Cannon Attorney-William H. Myers and Joseph W. Brown [57] ABSTRACT Certain hydrogenated or saturated block copolymers having at least three blocks are highly effective viscosity index improving additives for mineral oils and are especially effective at elevated temperatures.

6 Claims, No Drawings BLOCK COPOLYMERS AS VISCOSITY INDEX IMPROVERS FOR LUBRICATING OILS This invention is concerned with lubricating compositions. More particularly, it is directed to lubricating oils having substantially improved shear stability and viscosity index.

Mineral lubricating oils have been modified by a vast array of additives for purposes of improving viscosity index, thermal stability, oxidation stability, detergency, and other properties. The viscosity index is highly important especially in multigrade oils to provide lubricating oil compositions having much flatter viscosity-temperature curve than the unmodified oil. It is especially vital that the lubricating oil compositions exhibit specified maximum viscosities at relatively low temperatures and specified minimum viscosities at relatively elevated temperatures. The viscosity index of mineral lubricants has been altered by the presence of high molecular weight polymeric additives such as polymethacrylates. However, apparently due to their high molecular weight, such additives are found to be sensitive to thermal or oxidative degradation and particularly to degradation under the degree of shear which is experienced during dynamic utilization of the lubricant in machinery and the like. Thus while the additive may be promising as a viscosity index improver in mineral lubricants prior to its exposure to shear forces, in many instances it is found that many of the compositions rapidly lose their initial beneficial properties and gradually revert to the undesirable viscosity-temperature relationships of unmodified oil. The search for new and improved types of viscosity index improvers is not aided by observing the effect of various potential additives in mineral fuels such as fuel oil, gasoline, kerosene and the like, since the demands made by such fuels have little if any relationship to the viscosity index requirements and physical conditions encountered with lubricating oil compositions. Many materials are useful, for example, as pour point reducing agents in fuels but have little if any effect upon the viscosity index thereof. Moreover, the use of many viscosity index improving agents in lubricating oils have substantially no beneficial effect upon the properties of fuel oils. Consequently, the arts surrounding these two separate fields has grown up independently of each other.

In addition to the properties of improving viscosity index and of being stable under conditions of high shear, it is necessary for any potential lubricating oil additives to have two other important properties, namely, compatibility with the lubricating oil and stability under conditions of oxidation which would be reasonably expected to be encountered under conditions of storage and use of such compositions.

It is an object of the present invention to provide improved mineral oil compositions. It is a particular object of the invention to provide lubricating oil compositions having substantially improved viscosity index properties. It is a further object of the invention to provide multi-grade lubricating oil compositions exhibiting substantially improved viscosity index as well as a high resistance to degradation of such improved properties under shear encountered during lubricating operations. lt is a further object of the invention to provide an improved method of lubrication. Other objects will become apparent during the following detailed description of the inventron.

Now, in accordance with the present invention, improved lubricating compositions are provided comprising a mineral oil and as a viscosity index improver therefor 075-5 .0 percent by weight of a block copolymer having at least three polymer blocks C and D, blocks C being hydrogenated monovinyl arene polymers, having average molecular weights of 5,000-50,000 and blocks D are polymer blocks of the group consisting of alpha olefin polymers and hydrogenated conjugated diene polymers wherein at least 50 percent of the original olefinic double bonds have been reduced by hydrogenation said block having average molecular weight between about 10,000 and 1,000,000.

Still in accordance with the present invention, an improved process of lubrication is provided comprising lubricating relatively moving metallic surfaces with the mineral oil compositions just described.

The mineral oils particularly contemplated for use in the present compositions particularly useful for engine lubricants generally have viscosities between about 150 and about 250 SSU at F. and generally are described as having SAE grades of 5-50. These are usually mineral oil distillates but may comprise or contain mineral oil residuals as long as the compositions has lubricating properties. While low viscosity index mineral lubricants are employed, it is much more desirable to utilize those having viscosity indexes between about and about 200, the higher the better, especially when multigrade lubricants are being compounded. Multi-grade lubricams are especially contemplated such as 10/30 or 20/40 oils either for summer or winter use. Oils suitable in greases, hydraulic oils, open gear lubricants and other lubricating situations also are contemplated.

In accordance with the present invention, the essence thereof lies primarily in the discovery that certain and only certain hydrogenated block copolymers are not only compatible with mineral oillubricants but-also substantially improve the viscosity indexes thereof and exhibit a surprising and unaccountable degree of stability under the rates of shear expected and encountered during lubricated operations. Moreover, due to the substantial degree of hydrogenation as more particularly described hereinafter, the polymers are especially stable even under oxidizing conditions. Furthermore, one of the aspects of the present invention lies in the relative low molecular weight of the polymers utilized therein as compared with the substantially higher molecular weight polymers utilized by the prior art. The stability of the polymers of this invention under degrees of thermal stress, oxidative influences and particularly under shear is not only highly unexpected but essential to their success in lubricating processes. Contrary to the scission which may occur when a random copolymer or homopolymer degrades, the permanent scission of a block copolymer will result in catastrophic degradation of its physical properties as well as of its molecular structure.

It is essential for the block copolymers to be compatible with the mineral oils in which they are utilized if they are to be successful viscosity index improving additives. For this purpose its necessary to carefully select the block molecular weights and type of block which in the entire structure of the block copolymer will be compatible with the lubricating oil. This may of course vary to a certain degree depending upon the aromatic and aliphatic contents of such oils. However, the generic aspect of the present invention broadly contemplates the several types of block copolymers which will be suitable in this respect.

The block copolymers as represented hereinbefore may be either linear or branched in its structure, the linear species represented by the general formula C(DC),, while the branched configuration is represented by the general configuration CD(DC),,. In the above formulae, the subscript n is a whole integer, preferably 1 to 5, and C is a hydrogenated monovinyl arene polymer block wherein at least 50 percent of the original aromatic double bonds are reduced by hydrogenation, having an average molecular weight between about 5,000 and 50,000. Blocks D are either aliphatic mono-alphaolefin polymer blocks or hydrogenated conjugated diene polymer blocks in which at least 50 percent of the original diene unsaturation is reduced by hydrogenation; said blocks D having average molecular weights of 10,000-1,000,000, preferably 15,000-200,000. The most simple linear block copolymer of this type is hydrogenated polystyrenehydrogenated polyisoprene-hydrogenated polystyrene or polyvinylcyclohexane-EPR-polyvinylcyclohexane and their counterparts wherein the center block is hydrogenated polybutadiene, preferably having a 1,2 content of 30-60 percent. The term EPR refers to an ethylene/propylene random copolymeric block. The branched counterparts of these linear block copolymers are normally obtained by coupling reactions which involve coupling agents having a functionality higher than two. The preparation of these block copolymers does not form a part of the present invention. Also contemplated are block copolymers having the hydrogenated diene polymer blocks on the ends of the chains as in hydrogenated polyisoprene-hydrogenated polystyrene-hydrogenated 566w base oil blend. The table below shows the four types tested in the lubricating oil:

polyisoprene.

The conjugated dienes which may be employed in forming TABLEI the block polymers to be later hydrogenated include especially butadiene and isoprene as well as mixtures thereof. The Block Lengths monovinyl arenes to be used in the block copolymer forma- Sample p tion include. particularly styrene as well as ring alkylated styrenes'and their mixtures. lf block copolymers are formed 10 1 polyvinylcyclohexanbhydmgenated 13444 5 incorporating alpha olefin blocks as the blocks D, the polybutadienepolyvinylcyclohexane al|t2d species include ethylene, propylene, and mixtures 2 p t y ysi yi g p 134445 In addition to homopolymer blocks or copolymer blocks of 3 m g'lfili fi'f 23.40243 dienes in any single block, copolyrner blocks of monovinyl p y p -p y y y n arenes with conjugated dienes may be employed in any or all 4 plysltyrne'hydrgnated 23402-23 of blocks C or D as described above. In this case the blocks p0 ystyrene should predominate prior to hydrogenation in the previously described monomers. A typical polymer of this type prior to hydrogenation will have the structure polystyrene-random Samples 2 and 4 were found to be incompatible with the B' T term SBR, refers a Styrene/bu lubricating oil to the extend that lumpy gels were formed at tadlene copolymenc block" 0.2 percent by weight of polymer in the oil. Their contribublock copPlymers are hydrogenated to reduce their tions to the viscosity of the oil were too small for. them to be olefinic unsaturation by at least 50 percent and preferably at 25 useful as VI improves. The remaining two polymers f d least 80 Percent of the original olefin: double bondssuitable solutions in oil and their concentrations were adjusted Moreoven y of the block copolymers having mom than a to give kinematic viscosity values measured at 210 F. com- Single monovinyl arene P y block e hydrogenated so as parable to that of the comparative terpolymer additive. to reduce the Original aromatic double bnds y a! least 50 The shear stabilities of the polymers in solution were deterp and Preferably at least 80 Percent Hydrogenation is mined by the kinematic viscosity loss of the solution measured Preferably carried n in Solution utilizing either nomogene' at 210 F. resulting from polymer degradation in a Raytheon ous or heterogeneous catalysts. If both aromatic and olefinic Sonic shear apparatus Thi cc Samples were d d d at double bonds are to be reduced then relatively stringent 100 F f 30 minutes at a frequen y f 9,170 cycles per hydrogenation Conditions y be p y Preferably, second and kinematic viscosities of the solutions were meaever, the more readily saturated olefinic double bonds are first 3 5 Sured b f d aft shean Th viscosity l 210 F reduced at relatively mild hydrogenation conditions after tributable to polymer degradation is given by the equation which temperature and pressure may be increased so as to effectively cause reduction of aromatic unsaturation. Catalysts V, V F such as cobalt or nickel salts or alkoxides reduced with aluloss: V V X 100 minum alkyl compounds are employed preferably as catalysts. a..- Suitablecatalysts include nickel acetate, nickel octoate, or where V,, V and V refers to viscosities of the initial solution nickel acetyl acetonate reduced with aluminum alkyl combefore shear, of the solution after shear, and of the base oil pounds such as aluminum triethyl, aluminum triisobutyl, or blend less VI improver, respectively. The results are given in aluminum trioctyl. the table below.

TABLE II Viscosity before shear Viscosity after shear Percent weight 100 210 100 210 Percent Sample polymer 0 (cp) (SUS) (SUS) 0 (cp) (SUS) (SUS) loss Base blend 1800 187 48.0 1820 186 47.6 2.1 2100 377 75.3 2050 271 59.1 59 1.3 2800 370 68.0 2800 353 65.6 12 1.1 2400 342 65.7 2450 325 63.1 15

The following examples illustrate the benefits obtained and The viscosity measurements in the table are to be compared the limits of the present invention. not only with each other but also with the specifications which a l0W/30 motor oil must meet. It must have a maximum EXAMPLE I viscosity at 0 F. of 2,400 cp and a minimum viscosity of 210 60 F. of58 SUS. Lubricating oil Pans igo Thus it is clear from the table that the commercially utilized Carbonaled Ca Sulfonales lcomparative terpolymer has suitable low temperature viscosi- Polybutenyl succinimide of polyethylene y properties. However, it loses much of its high temperature 21:? silk dimiophosphates 8:? viscosity due to polymer shear, apparently because of its very lseoei i m OJ high molecular weight. The two polymers of this invention, Silicone Oil 10 ppm however, both show much better stability toward shear then For comparison, a solution of a well-known viscosity index improving additive at a concentration of 2.1 weight percent was prepared. This additive was a random terpolymer about 800,000 molecular weight composed of 60 percent lauryl methacrylate, 35 percent stearyl methacrylate, and 5 percent 2-methyl-5-vinylpyridine. Four different types of block copolymers were investigated, attempts being made to form homogeneous dispersions of the block copolymers in the the comparative terpolymer, possibly not only because of their stable structure but because of their relatively low molecular weights. Despite their low molecular weights, they give thickening power at 210 F. comparable to that of the commercially utilized comparative terpolymer and at comparable or lower concentrations. 1

In similar tests a hydrogenated random SBR rubber was tested for its shear degradation. It was found that it lost about two-thirds of its thickening power in similar tests.

1 claim:

1. A lubricating composition comprising a mineral oil and as a viscosity index improver therefor 0.75-5 percent by weight of a hydrogenated block copolymer having at least three essentially uniform blocks C and D, blocks C being hydrogenated polymer blocks of styrene wherein at least 50 percent of the original aromatic unsaturation has been reduced by hydrogenation of the polymer, said blocks C hav ing an average molecular weight between about 5,000 and 50,000, and blocks D being hydrogenated polymer blocks of conjugated dienes wherein at least 50 percent of the original unsaturation has been reduced by hydrogenation of the polymer, said blocks D having average molecular weights between about 10,000 and 1,000,000.

2. A composition according to claim 1 where the oil is a lubricating oil.

3. A lubricating composition according to claim 1 comprising a mineral lubricating oil and 1.0-4.5 percent by weight of a three-block copolymer of a hydrogenated polystyrene block and a hydrogenated polyisoprene block.

4. A lubricating composition according to claim 1 comprising a mineral lubricating oil and l.04.5 percent by weight of a block copolymer of alternating hydrogenated polystyrene blocks and hydrogenated conjugated diene polymer blocks.

5. A lubricating composition according to claim 1 having a viscosity no greater than about 2,600 cp at 0 F. and a viscosity ofat least about 50 cp at 210 F.

6. A lubricating composition according to claim 1 containing in addition a stabilizing proportion of lubricating oil detergent. 

2. A composition according to claim 1 where the oil is a lubricating oil.
 3. A lubricating composition according to claim 1 comprising a mineral lubricating oil and 1.0-4.5 percent by weight of a three-block copolymer of a hydrogenated polystyrene block and a hydrogenated polyisoprene block.
 4. A lubricating composition according to claim 1 comprising a mineral lubricating oil and 1.0-4.5 percent by weight of a block copolymer of alternating hydrogenated polystyrene blocks and hydrogenated conjugated diene polymer blocks.
 5. A lubricating composition according to claim 1 having a viscosity no greater than about 2,600 cp at 0* F. and a viscosity of at least about 50 cp at 210* F.
 6. A lubricating composition according to claim 1 containing in addition a stabilizing proportion of lubricating oil detergent. 